Filters for hydraulic fluid are commercially available and are widely used, for example, in hydraulic systems in system branches through which hydraulic oils as a fluid flow. For a filter design which is stable under pressure, the filter typically has a support tube, preferably of plastic material, provided with perforations and supporting the filter medium used opposite the intended throughflow direction of the fluid. Generally two end caps have the filter medium therebetween and, if necessary, the plastic support tube extends between them. The end caps are likewise preferably made of plastic materials; in particular, the plastic support tube is made as an injection-molded component. In addition, there are also filter solutions in which the support tube is formed from a perforated metal jacket or in which the support tube can also be completely omitted as a support element for the filter medium.
Since, for fixing to the end caps, the filter medium is generally cemented to them in the region of its free ends, the cement used, often in the form of epoxy resin or PU adhesive, produces a type of insulating layer between the filter medium used, the two end caps, and/or the support tube. The insulation effect is further enhanced by the support tube preferably always being made longer than the actual filter medium (mesh pack) so that forces cannot act on it in the longitudinal direction. If this application of forces is not prevented, it cannot be precluded that, as a result of the pressure stresses arising when the fluid is flowing through the filter medium, the filter medium will be damaged. Adequate filtration performance can no longer be ensured.
Electrostatic charges, particularly of the filter medium, occur during fluid passage with possible particulate fouling on the filter medium due to the indicated insulation structure, dictated by insulation layers between the filter medium, the end caps and the support tube. As a result of the potential differences produced in this way within the filter element, discharges can suddenly occur between statically charged filter element parts, especially in the form of the filter medium, and the electrically conductive components, particularly in the form of the typically metallic filter housing in which the filter medium is held, with the result that spark discharges occur. This occurrence must be considered critical with respect to flammability of the fluid media which are to be filtered, such as hydraulic oil, heavy oil fuels, such as diesel fuels, or the like. The spark discharges can also lead to damage to the oil and to sensitive filter medium material. It has furthermore been shown that due to electrostatic charges of oil media, they can age more quickly and must be replaced earlier as part of maintenance.
In order to eliminate these disadvantages, in the generic filter according to DE 10 2004 005 202 A1, it was proposed that at least one of the end caps of the filter and/or at least one end region of the filter medium have a contact-making device and/or the respective end cap itself or parts of it be made electrically conductive for purposes of dissipation of the electrostatic charges which occur especially in operation of the filter. In practice, the known solution, in which the potential differences and charges which arise can be dissipated exclusively by way of the end caps, especially the lower end cap, into the grounding point formed by the housing, should be improved.